Method of manufacturing a distributed brass reflector type semiconductor laser

ABSTRACT

Disclosed are a distributed Bragg reflector type semiconductor laser and a method of manufacturing such a laser a high yields, in which the upper surface of an active waveguide is covered by an external waveguide, the external waveguide at side portions thereof, the external waveguide is coupled with the edge surfaces of the active waveguide without any gap remaining, and the coupling ratio of the active waveguide and external waveguide is high.

This is a division of application Ser. No. 239,120 filed Aug. 31,. 1988,U.S. Pat. No. 4,914,670.

FIELD OF THE INVENTION

The present invention relates to a distributed Bragg reflector typesemiconductor laser and a method of manufacturing such a laser.

A distributed Bragg reflector type semiconductor laser of the typecalled BIG-DBR is one type of semiconductor laser which, because of itsexcellent performance, is at present regarded as a superior class oflaser.

This type of laser was proposed by Tohmori et al. in "Japanese Journalof Applied Physics", Vol. 24, No. 6, 1985, pages 399 to 401. The presentinvention relates to an improvement of this laser.

DESCRIPTION OF RELATED BACKGROUND ART

With a light source having a wavelength of a band of 1.55 μm which isintended to provide a form of light communication over long distanceswith a large capacity, it is required that it has a single longitudinalmode which is stable, that it can be monolithically integrated withother functional devices with ease, and so on.

A dynamic single mode laser is a suitable form of laser which cansatisfy these conditions.

Dynamic single mode laser is include distributed feedback type lasers(DFB), distributed Bragg reflector type lasers (DBR), compound resonatorlasers, and so on.

Among these, the distributed Bragg reflector type laser is of particularinterest.

In a distributed Bragg reflector type laser, a diffraction grating isformed on a waveguide and used as a distributed Bragg reflector.

This kind of laser has many advantages such as the fact that the stablesingle longitudinal mode operation can be easily maintained upon highspeed modulation, that the laser can be monolithically integrated withother functional devices with case, that a length of a laser resonatorcan be set to a short cavity, and so forth.

Distributed Bragg reflector type lasers are mainly classified as DBR-ITGtype lasers which have an integrated twin-waveguide structure andDBR-BJB type lasers which have a direct coupling structure.

The first of these type is the Distributed Bragg reflector integraltwin-guide type laser which has two waveguides. An external waveguide isextended to the outside. An active waveguide is formed over thatwaveguide through a buffer layer. The distributed Bragg reflectorsurface is provided on the external waveguide.

This kind of laser involves the problem that part of the light isreflected between the active waveguide and the external waveguide. Thatis, there is a problem since the active waveguide is formed over theexternal waveguide and the coupling state therebetween is thereforeinferior and the reflection loss in the coupling portion is large.

In a DBR-BJB type laser, both the active waveguide and the externalwaveguide are arranged in an almost rectilinear configuration. Namely,neither of the waveguides exist in the vertical direction but arearranged in the same plane; thus the reflection loss in the couplingportion is small.

In actual practice, however, a level difference can easily occur at theboundary between the two waveguides. There are certain drawbacks in thatthe active waveguide and external waveguide are incompletely coupled inthe level difference portion and the light is reflected by this leveldifference portion.

Due to this reflection loss, the coupling efficiency between thesewaveguides is low. Consequently, the conventional DBR type lasersexperience problems in that multi-mode oscillation can easily occur andthe differential quantum efficiency is low.

With a view to solving these problems, Suematsu, Arai, Tohmori, et alinvented the distributed Bragg reflector type (BIG-DBR type)semiconductor laser, their patent application therefor (Japanese PatentApplication No. 60-12181, JP-A-61-171190) having been filed on Jan. 25,1985, and laid open on Aug. 1, 1986.

The laser has a waveguide structure wherein an active waveguide issurrounded by an external waveguide.

In the central portion of this structure, the external waveguide isprovided over the active waveguide (which is opposite to the case of theITG type laser), while in the side portions, the external waveguide isdisposed over an extended surface of the active waveguide. Thus theactive waveguide and external waveguide are coupled in two directions,i.e., in the vertical direction and in the direction of the horizontalsurface, whereby the coupling efficiency is improved. In actualpractice, however, there is a serious problem with respect to the yieldsobtainable with this laser.

This point will now be explained with reference to FIGS. 8 to 11, whichare cross-sectional views showing the manufacturing steps.

An InGaAsP active waveguide 12 and an InP depression layer 13 aresequentially formed on an p-type InP (100) substrate 11 by a liquidphase epitaxial method.

Next, the InP depression layer 13 and InGaAsP active waveguide layer 12are etched with the central portions retained. At this time, a part ofthe upper surface of the InP substrate 11 is also etched.

A grating 16 is formed on the upper surface of the exposed InP substrate11. FIG. 8 is a cross-sectional view showing the state of a laser formedby the foregoing steps.

As will be understood from FIG. 8, an edge surface U of the InPdepression layer 13 and an edge surface W of the InGaAsP activewaveguide 12 form one flush surface. This surface will be parallel to orslightly inclined from a (110) cleavage plane.

An InGaAsP external waveguide layer 14 is then formed on the depressionlayer 13 by the liquid phase epitaxial method.

It is desirable that the external waveguide layer 14 is so formed as touniformly cover the upper surfaces of both the substrate 11 and thedepression layer 13.

However, there is a problem in that the InGaAsP external waveguide layer14 is not properly formed in the immediate vicinity of the InGaAsPactive waveguide layer 12 and InP depression layer 13.

FIG. 9 shows such an improper state.

The external waveguide layer 14 grows in an epitaxial manner on the InPsubstrate 11 and also on the depression layer 13.

However, in the regions near the edge surfaces U of the depression layer13 and the edge surfaces W of the active waveguide 12, the layer 14hardly grows at all and gap portions G remain.

FIG. 10 is a cross-sectional view showing a state similar to FIG. 9. Theedge surfaces U and W of the depression layer and active waveguide areslightly inclined in the direction toward the (111) plane. Actually,although it is unclear whether the edge surfaces U and W are inclined inthe direction of the (110) plane or the (111) plane, in either case theexternal waveguide layer fails to grow in a state in which it intimatelyconnects with the edge surfaces W and U.

FIG. 11 shows a special example of FIG. 9. It will be understood fromthis diagram that the InGaAsP external waveguide layer 14 iscontinuously formed without any gap at the edge surfaces U and W, buteven in this continuous state dimples 18 remain.

In the case of the example shown in FIG. 11 the InGaAsP externalwaveguide layer 14 is continuously formed on the InP depression layer 13and InP substrate 11, and the active waveguide and external waveguideare continuous with the edge surfaces W. However, although both thewaveguides are coupled and the light in the active waveguide can betransmitted to the external waveguide, it is difficult to obtain aproper coupling state between them due to the influence of the dimples.

Lasers with structures of the described above have in fact beenmanufactured and, as shown in FIGS. 9 and 10, the external waveguide isinterrupted on both sides of the active waveguide with gaps remaining.Almost all of the manufactured lasers of this type are like this. Sincethe coupling efficiency is 0, these products cannot function as lasers.There have been cases where lasers of the type shown in FIG. 11 havebeen produced at a low yield. However, the yield is only a few % and itis difficult to manufacture products which can be used as lasers inpractice.

The reason why the external waveguide is cut each side of the activewaveguide and depression layer is unclear. A possible reason is that theedge surfaces U of the depression layer and the edge surfaces W of theactive waveguide are in the (111) mesa directional plane. There is apossibility that such edge surfaces are oblique surfaces as shown inFIG. 10.

It is a known fact that in liquid phase epitaxy crystal growth hardlyever occurs in the (111) mesa directional plane of InP.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method ofmanufacturing at a good yield a laser of the BIG-DBR type in which theexternal waveguide is coupled with the active waveguide at the edgesurfaces without any gap remaining in such as laser in which an externalwaveguide covers the upper surface of an active waveguide and in whichthe external waveguide extends on the same surface as the activewaveguide at its side portion.

Another object of the invention is to provide a BIG-DBR type laserhaving high coupling efficiency between the active waveguide and theexternal waveguide.

The inventors of the present invention have made great efforts by trialand error to find a method of securely coupling the active waveguidelayer and external waveguide layer in the horizontal direction.

They have found that the waveguides can be coupled with certainty by amethod in which the edge surfaces U of a depression layer are extendedon both sides so as to project beyond the edge surfaces W of theexternal waveguide layer.

FIG. 1 shows a vertical sectional view of a basic type of distributedBragg reflector type laser according to the present invention. FIG. 1 isa diagram of the principle; in practice other semiconductor layers arealso laminated.

The structure of this laser will be described with reference to FIG. 1.

An active waveguide 3-1 and a depression layer 4-1 are sequentiallyformed on a substrate 1-1 and grown in an epitaxial manner similar tothe conventional lasers shown in FIGS. 8 to 11.

When the side portions of the active waveguide layer 3-1 and depressionlayer 4-1 are removed by etching in this invention, the lower edgesurfaces of the active waveguide layer 3-1 are deeply etched by aselective etching method.

Namely, the lower edge surfaces W of the active waveguide layer 3-1 arelocated at backward positions and the upper edge surfaces U of thedepression layer 4-1 are projected forward beyond the edge surfaces W.

Such a structure is called an overhang in the field of architecture.This structure of the active waveguide layer 3-1 and depression layer4-1 is hereinafter referred to as an overhang in this specification.

The projection length L of the edge surface U beyond the edge surface Wis called an overhang length. The overhang length is set at about 0.2 to1.0 μm.

With such an overhang structure, the edge surfaces of the activewaveguide layer 3-1 and depression layer 4-1 do not become a flushsurface. A rectangular space Σ is formed whose surfaces in the threedirections are surrounded by a back surface R of the depression layer4-1, edge surface W of the active waveguide layer 3-1, and an uppersurface Q of the substrate 1-1.

Due to the existence of the space Σ, when an external waveguide layer7-1 is formed by a liquid phase epitaxial method, no air gap i formedbetween the external waveguide layer 7-1 and the active waveguide layer3-1.

It should be noted that the solution used as the material for formingthe external waveguide 7-1 enters this internal space Σ by the liquidphase epitaxial method and this material grows in an epitaxial manner inthis space. It is considered that, as shown in FIGS. 8 to 11, in thecase of configuration wherein the active waveguide layer 3-1 is exposedto the outside, the InGaAsP eiptaxial layer will grow easily. However,in general, the mechanism of the epitaxial growth can be described bywhat is called a Kossel model with the growth progressingtwo-dimensionally by supplying steps and kinks.

With the structure of the short active waveguide layer 3-1 as shown inFIG. 1, by supplying the steps (growth nuclei), the solution comes intocontact with the edge surfaces W of the active waveguide layer, andtwo-dimensional epitaxy in the lateral direction occurs fairly easily.Thus, the external waveguide layer can grow smoothly.

The overhang length is set at about 0.2 to 1.0 μm as mentioned above. Ifit is 0.2 μm or less, a gap G will be formed between the activewaveguide layer 3-1 and the external waveguide layer 7-1 in a mannersimilar to the conventional examples. If it is 1.0 μm or more, the edgeprojecting portions of the depression layer 4-1 will be broken. Sincethe depression layer 4-1 is as thin as about, e.g., 0.1 to 0.3 μm, itwould be broken if the overhang length were to be set at 1.0 μm or more.If the depression layer is broken, the advantages of the presentinvention are of course lost.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of the principle of a distributed Bragg reflectortype semiconductor laser according to the present invention;

FIG. 2 is a cross-sectional view showing a state in which a bufferlayer, an active waveguide layer, and a depression layer are eiptaxiallygrown on an InP substrate;

FIG. 3 is a cross-sectional view showing the state after the etching ofthe depression layer and active waveguide layer in the form of a stripeafter formation of an SiN film;

FIG. 4 is a cross-sectional view showing the state in which adistributed Bragg reflector is formed on the buffer layer;

FIG. 5 is a cross-sectional view of the state after selective etching ofthe edge surfaces of the active waveguide layer in the lateraldirection;

FIG. 6 is a cross-sectional view of the state after epitaxial growth ofan external waveguide layer and a clad layer;

FIG. 7 is a cross-sectional view showing the state in which a P-sidedelectrode and an N-sided electrode are formed;

FIG. 8 is a cross-sectional view showing the state in which an activewaveguide layer and a depression layer are formed by epitaxial growth ona substrate in a conventional method of manufacturing a BIG-DBR typelaser and, thereafter, the active waveguide layer and depression layerare etched in the form of a stripe;

FIG. 9 is a cross-sectional view showing an example in which, when anexternal waveguide layer is formed by epitaxial growth on an activewaveguide layer and a depression layer in a conventional method ofmanufacturing a BIG-DBR type laser, air gaps are formed between theactive waveguide layer and the external waveguide layer, the uppersurface thus becoming discontinuous;

FIG. 10 is a cross-sectional view showing an example in which air gapsare formed between the active waveguide layer and the external waveguidelayer in a manner similar to FIG. 9 but in which the edge surfaces ofthe active waveguide layer and depression layer are inclined; and

FIG. 11 is a cross-sectional view showing an example in which the activewaveguide layer and external waveguide in a conventional method ofmanufacturing a BIG-DBR type laser are connected but dimples are formedtherebetween.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A method of manufacturing a distributed Bragg reflector typesemiconductor laser according to the present invention will be describedin detail hereinbelow with reference to FIGS. 2 to 8.

(1) A p-type InP buffer layer 2, an InGaAsP active waveguide layer(non-dope) 3, and an n-type InP depression layer 4 are sequentiallyformed and grown on a p-type InP substrate 1 by liquid phase epitaxy orvapor phase epitaxy. This growth step relates to the first epitaxy shownin FIG. 2.

An InGaAsP anti-meltback layer can also be inserted between thenon-doped InGaAsP active waveguide layer 3 and the n-type InP depressionlayer 4 so as to make the interface smooth.

The p-type InP buffer layer 2 corresponds to the portion designed toform a diffraction grating and has a thickness of 1 to 4 μm.

The non-doped InGaAsP active waveguide layer 3 corresponds to theportion which emits the light when excited by a current supply. Thislayer is as thin as about 0.1 to 0.4 μm.

The n-type InP depression layer 4 protects the layer 3; however, itsthickness is limited to about 0.05 to 0.3 μm so that an overhangstructure is formed. It is desirable to set this thickness at a value of0.1 μm or more so that the overhang has sufficient strength. However, itis preferable to make the depression layer 4 as thin as possible inorder to raise the coupling efficiency of the waveguides.

InGaAsP is a mixed crystal in which a mixture ratio of In and Ga andmixture ratio of As and P are used as parameters. However, these twoparameters cannot be freely set because the appropriate lattice matchwith InP must be realized. The degree of freedom is 1.

Since one degree of freedom is left, a band gap or the like can beselected by changing the mixture ratio.

In this specification, mixture ratios x and y or the like are omitted toavoid the complexity involved when they are written as suffixes.

(2) Next, a stripe-shaped SiN film 5 is formed on the n-type InPdepression layer 4.

The portion of the depression layer 4 which is not covered by the SiNfilm 5 is etched by an etching liquid of the HCl-based.

Next, the portion of the non-doped InGaAsP active waveguide 3 which isnot covered by the SiN film 5 is etched by an etching liquid of the H₂SO₄ -based.

In this case, a selective etching process is used. An etching liquid ofthe H₂ SO₄ -based does not act on the n-type InP depression layer 4. Theetching liquid of the HCl-based does not act on the non-doped InGaAsPactive waveguide 3.

The edge surfaces W and U of the active waveguide layer 3 and depressionlayer 4 become a flush surface through the above etching process. FIG. 3shows the steps taken to reach this state. The width of a stripes isabout 200 to 400 μm.

(3) A diffraction grating serving as a distributed Bragg reflector 6 isformed on the exposed p-type InP buffer layer 2 by using an interferenceexposing method. This state is shown in FIG. 4.

The edge surfaces of the SiN film 5, n-type InP depression layer 4, andnon-doped InGaAsP active waveguide layer 3 become a flush surface. Thedistributed Bragg reflector 6 is extended to the right and left from theedge surface of the active waveguide layer 3.

The grating interval of the reflector 6 is determined by the lightemitting wavelength of the laser.

(4) Next, the active waveguide layer 3 is etched inwardly from the edgesurface by using the h₂ SO₄ -based etching liquid in which the n-typeInP depression layer 4 is used as a mask. The H₂ SO₄ -based etchingliquid consists of H₂ SO₄, H₂ O₂ and H₂ O and does not ct on the n-typeInP depression layer 4, which allows the selective etching to beaccomplished. The etching speed can be controlled by changing thecomponent ratio of the etching liquid. For example, the etching speedmay be set to about 0.1 μm/minute and, generally, to a value within therange of 0.02 to 0.2 μm/minute.

The etching depth in the lateral direction corresponds to the overhanglength L. This dimension is set to about 0.2 to 1.0 μm as mentionedabove.

This state is shown in FIG. 5.

(5) Next, an n-type InGaAsP external waveguide layer 7 is grown on theupper surfaces of these layers by liquid or vapor phase epitaxy.

The InGaAsP material efficiently enters the space Σ of the overhangstructure and crystal grows continuously from the edge surfaces W of thenon-doped InGaAsP active waveguide layer 3.

The space Σ is a long narrow space in the lateral direction in which thedepth is set at 0.2 to 1.0 μm and the height at 0.1 to 0.4 μm. Thesolution or gas flow can efficiently enter this space and the epitaxialgrowth occurs from the edge surfaces W without leaving any air gap.

In this manner, the n-type InGaAsP external waveguide layer 7 becomes alayer which covers both the n-type InP depression layer 4 and thedistributed Bragg reflector 6 and extends continuously. The thickness ofthe external waveguide layer 7 on the upper surface of the depressionlayer 4 is about 0.1 to 0.4 μm.

Additionally, an n-type InP clad layer 8 is grown on the externalwaveguide layer 7 by liquid or vapor phase epitaxy.

The thickness of n-type InP clad layer 8 is set at about 1.0 to 3 μm.

FIG. 6 shows a product formed using the above steps.

The band gap of external waveguide layer 7 is set to wider than that ofthe active waveguide layer 3. This is because, since the mixed crystalratio of InGaAsP has one freedom parameter, the band gap can be adjustedby changing the mixed crystal ratio.

The foregoing relation of the band gaps is set so that the lightgenerated by the active waveguide layer 3 is not absorbed by theexternal waveguide layer 7.

(6) Further, a P-sided electrode 9 is formed under the InP substrate 1and an N-sided electrode 10 is formed on the InP clad layer 8. TheN-sided electrode 10 is formed as a stripe to thereby limit the currentdistribution.

The distributed Bragg reflector type semiconductor laser of theinvention is manufactured in this manner.

After that, a wafer is scribed to obtain device chips. These chips aremounted on a package. When a current flows between the P- and N-sidedelectrodes, light is generated in the active waveguide layer 7 due tothe stimulated emission. This light progresses to the external waveguidelayer 7 in the same plane and part of the light is reflected by thefunction of the distributed Bragg reflector 6. By repeating thisreflection, the laser power is raised. Part of the light is emitted tothe exterior from the external waveguide layer 7 as shown by arrows Aand B in FIG. 7.

In the distributed Bragg reflector type semiconductor laser manufacturedas described above, the non-doped InGaAsP active waveguide layer 3 andn-type InGaAsP external waveguide layer 7 are effectively coupled. Theconsistency between the two waveguides is good.

A laser of the basic type and a manufacturing method thereof have beendescribed above.

The laser described above is manufactured by a method wherein the n-typedepression layer, the n-type external waveguide layer, and n-type cladlayer are formed by epitaxial growth on the p-type InP substrate.

It is also possible to grow a p-type depression layer, a p-type externalwaveguide layer, and a p-type clad layer on the n-type InP substrate byreversing the electrical polarities.

The buffer layer 2 is provided to form a diffraction grating. Adiffraction grating can be also formed on the surface of the substrateexcluding the buffer layer.

The structure of the distributed Bragg reflector type semiconductorlaser of the invention can also be used together with an embeddedstructure of the type generally used (for instance, refer to Asahi etal, "Journal of Applied Physics", Vol. 55(3), 1984 pages 656 to 659).

The above description relates to the distributed Bragg reflector typelaser of the InGaAsP system formed on an InP substrate. However, theInvention can be also applied to a laser of the GaAs system formed on aGaAs substrate.

The method of manufacturing the distributed Bragg reflector typesemiconductor laser of the invention will be more specifically explainedby concrete example.

1) As shown in FIG. 2,

p-type InP buffer layer 2 2 μm

non-doped InGaAsP active waveguide layer 3 . . . 0.15 μm

n-type InP depression layer 4 0.1 μm

were grown on a p-type InP substrate by liquid phase epitaxy.

2) A SiN film pattern 5 having a width of 300 μm was formed as a stripe.This stripe pattern was formed in parallel with the (011) plane.

An n-type InP depression layer 4 was selectively removed by a HCl-basedetching liquid and the non-doped InGaAsP active waveguide layer 3 wasthen selectively removed by H₂ SO₄ -based etching liquid, therebyobtaining the state shown in FIG. 3.

3) The distributed Bragg reflector 6 was formed on the exposed p-typeInP buffer layer 2 by the interference exposing method whereby the stateshown in FIG. 4 was obtained.

4) The SiN film 5 was removed. The edge surfaces of the non-dopedInGaAsP active waveguide layer 3 were etched to a depth of 0.3 μm towardthe portions set back by using the H₂ SO₄ -based etching liquid, the InPdepression layer 4 being used as a mask.

The component ratios of H₂ SO₄, H₂ O₂ and H₂ O were adjusted so as toobtain an etching speed of 0.1 μm/minute.

During the etching process, the etching liquid was stirred in thedirection parallel to the direction of the stripe (FIG. 5).

5) On the distributed Bragg reflector 6 and buffer layer 2,

n-type InGaAsP external waveguide layer 7 . . . 0.2 μm (thickness on thedepression layer)

n-type InP clad layer 8 . . . 1.5 μm

were grown by liquid phase epitaxy. The band gaps were set as follows.

band gaps of the InGaAsP external waveguide layer 1.0 eV

band gaps of the InGaAsP active waveguide layer 0.8 eV

This state corresponds to FIG. 6.

6) Next, the P-sided electrode 9 and N-sided electrode 10 were formed.

The resultant semiconductor laser offered the following advantages.

1) Since the edge surfaces of the active waveguide layer are located atpositions set back behind the edge surfaces of the depression layer soas to form an overhang structure, no air gap remained between the edgesurfaces W of the active waveguide layer and the external waveguidelayer when the external waveguide layer was formed by epitaxial growth.

2) Since no air gap remains and the active waveguide layer and externalwaveguide layer are intimately connected, the product yield is high.Semiconductor lasers manufactured by the method of the invention wereevaluated in terms of yield on the basis of the coupling state of thewaveguides. It was found that a yield of 90% or better was obtainable.

In contrast, when the edge surfaces become a flush surface as shown inFIG. 8 to 11, the yield obtainable was close to 0, i.e., 0% to a few %.

The invention offers remarkably improved yields, a high product yieldbeing a very important factor for industry.

3) Since the coupling efficiency is high, the light emission efficiencyrises greatly when the laser of the invention is used as a semiconductorlaser.

4) The invention can also be applied to GaAs system semiconductorlasers, as well as InP system semiconductor lasers.

5) Since the laser of the invention is of the DBR type, it can bemonolithically integrated with other functional devices with ease.

The invention is not limited to semiconductor lasers; it can also beapplied to other devices such as optical ICs and the like.

Although the present invention has been illustrated and described withrespect to a preferred embodiment, various changes and modificationswhich are obvious to a person skilled in the art to which the inventionpertains are deemed to lie within the spirit and scope of the appendedclaims which define the invention.

What is claimed is:
 1. A method of manufacturing a distributed Braggreflector type semiconductor laser comprising the steps of:forming abuffer layer (2) on a p- or n-type semiconductor monocrystallinesubstrate (1) by an epitaxial growth method, said buffer layer (2)having the same electrical polarity as said substrate (1); forming byepitaxial growth a non-doped active waveguide layer (3) on said bufferlayer (2), or on said substrate (1) in the case where no buffer layer isinterposed; forming by epitaxial growth a depression layer (4) having anelectrical polarity opposite to that of said substrate (1) on saidnon-doped active waveguide layer (3); forming a stripe-shaped film (5)in a central portion of an upper surface of said depression layer (4);etching both the depression layer (4) and the active waveguide layer (3)while retaining the central portion covered by said film (5); forming adistributed Bragg reflector (6) serving as a diffraction grating onupper surfaces of the remaining depression layer (4) and the bufferlayer (2) which is exposed on both sides of said active waveguide layer(3) or on the upper surface of the substrate (1) by use of aninterference exposing method; etching edge surfaces of the activewaveguide layer (3) in the horizontal direction by using the depressionlayer (4) as a mask, thereby allowing edge surfaces (W) of the activewaveguide layer (3) to be set back by a distance of 0.2 μm to 1.0 μmbehind edge surfaces (U) of the depression layer (4); forming byepitaxial growth on the buffer layer (2) or the substrate (1) with saiddistributed Bragg reflector (6) formed thereon and on the depressionlayer (4) an external waveguide layer (7) having an electrical polarityopposite to that of the substrate (1); forming by epitaxial growth onsaid external waveguide layer (7) a clad layer (8) having an electricalpolarity opposite to that of the substrate (1); and forming electrodes(9) and (10) on the substrate (1) and said clad layer (8).
 2. A methodaccording to claim 1, wherein said epitaxial growth method is a liquidphase epitaxial growth method.
 3. A method according to claim 1, whereinsaid epitaxial growth method is a vapor phase epitaxial growth method.4. A method according to claim 2 or 3, wherein said substrate (1) is ap-type InP monocrystal, said buffer layer (2) is a p-type InPmonocrystal, said active waveguide layer (3) is a non-doped InGaAsPmixed crystal, said depression layer (4) is an n-type InP monocrystal,said external waveguide layer (7) is an n-type InGaAsP mixed crystal,and said clad layer is an n-type InP monocrystal.
 5. A method accordingto claim 4, wherein said depression layer (4) is selectively etched byan HCl-based etching liquid and said active waveguide layer (3) isselectively etched by an H₂ SO₄ -based etching liquid.
 6. A methodaccording to claim 5, wherein when the edge surfaces (W) of said activewaveguide layer (3) are etched in a lateral direction, the etching speedfalls within a range from 0.02 μm/minute to 0.2 μm/minute.